Chronexaline

Chronexaline is an experimental medical drug created by Starfleet Medical. It protects biomatter from tachyon radiation and other effects of time travel.

This drug was created sometime around 2404, at least in an alternate timeline, when the Doctor was one of the people working on it. Admiral Kathryn Janeway demanded 2000 milligrams of it for a "classified" mission, which later turned out to be an illegal trip through time to 2378, to ensure the USS Voyager's safe return 16 years prior to when it had happened in the unaltered timeline. (VOY: "Endgame").